1. Field of the Invention
The present invention relates to a method of welding a sealing glass to a laser glass and more particularly, it relates to a sealing method for a laser glass for increasing the bonding strength between the laser glass and a sealing glass formed thereon.
2. Description of the Prior Art
Recently, it has been reported that the characteristics of a laser can be improved by coating the outside of a laser glass disc with a glass to absorb the light scattered at the inside wall of the laser glass disc. The sealing glass used for this purpose must have properties such that the expansion coefficient of the sealing glass coincides with that of the laser glass as required in a conventional sealing glass and further the sealing glass should be capable of being welded at temperatures lower than the transition point, Tg, of the laser glass in order to not change the characteristics of the laser glass at welding and also the sealing glass should absorb light having definite wavelengths and satisfy sufficiently refractive index requirements. For example, in order to prevent light reflection at the inside wall of a Nd laser glass disc by coating such a sealing glass, the sealing glass must have the ability to absorb light of a wavelength of 1.06 microns, and to prevent light reflection even more effectively, it is desirable that the refractive index of the sealing glass is higher than the refractive index (about 1.6) of an ordinary laser glass but the difference between the refractive indices of the laser glass and the sealing glass be as small as possible. Moreover, in order to not change the characteristics of a laser glass at welding, the sealing glass must have a sealing temperature lower than the Tg (about 450.degree.C to 600.degree.C) of the laser glass, that is, the sealing glass must have a softening point, Sp, lower than the Tg of the laser glass by about 100.degree. C.
To meet these requirements, glasses have previously been discovered which have a low refractive index, the ability to absorb infrared rays, a low melting point, and an expansion coefficient .alpha. of 100 .+-. 20 .times. 10.sup..sup.-7 /.degree.C, as described in Japanese patent application No. 25,565/'73 and U.S. patent application Ser. No. 383,743, filed July 30, 1973 now U.S. Pat. No. 3,885,974 issued May 27, 1975. However, since a laser glass is exposed to a xenon flash lamp of 10 kilo joules in a glass laser oscillator, a sealing glass coated on a laser glass is used under quite severe conditions. Therefore, if the sealing glass can weld sufficiently without changing the characteristics of the laser glass, a concern is that the sealing glass may be destroyed due to the low melting point and also since it is desirable that the expansion coefficient .alpha. of the sealing glass match the expansion coefficient of the laser glass and also since the sealing glass has an expansion coefficient slightly lower than that of the laser glass so that a compressive stress is always applyed to the sealing layer or film, the bonding strength between the laser glass and the sealing glass is high. Also, a glass having a high melting point and a low expansion coefficient does not absorb infrared rays and thus an infrared absorber may be added to the glass. However, since the addition of the infrared absorber increases the expansion coefficient, it is desirable that the sealing glass used for the aforesaid purpose does not contain such an infrared absorber.